1. Field of the Invention
The present invention relates to an ion selective electrode and processes for the preparation of the same. More particularly, the invention relates to an ion selective electrode appropriately employable for the potentiometric determination of a potassium ion, a halogen ion or a carbonate ion, and processes for the preparation of the electrode.
2. Description of Prior Arts
The ion selective electrode is a device for the potentiometric determination of ion concentration in an aqueous sample, and a body fluid such as blood or serum. Its elemental structure is disclosed, for instance, in Japanese Patent Provisional Publication No. 52(1977)-142584 and U.S. Pat. No. 4,053,381. In more detail, the ion selective electrode has an integral structure comprising a support, an electroconductive metal layer (e.g., deposited silver metal layer), a layer of a water-insoluble salt of said metal (e.g., a layer of silver chloride), an electrolyte layer which comprises an electrolyte salt of a cation (e.g., potassium ion or sodium ion) with the same anion as the anion of the water-insoluble salt (for instance, potassium chloride or sodium chloride), and an ion selective layer.
In practically determining ion concentration by means of an ion selective electrode (i.e., half cell) having the above-described elemental structure, the following procedures are adopted: A couple of ion selective electrodes A & B are connected via a water-retainable bridge. On the ion selective electrodes A & B are spotted a standard liquid (reference liquid) and a liquid sample, respectively, and the potential difference between both electroconductive layers of the ion selective electrodes A & B are measured after lapse of a certain period of time. The measured potential difference is then compared against a calibration curve to determine the concentration of the electrolyte. Alternatively, a couple of ion selective electrodes insulated from each other by a scratched groove disclosed in Japanese Patent Provisional Publication No. 58(1983)-156848 can be used for the measurement in the same manner.
The ion selective electrode is composed basically of the above-stated simple structure and can be manufactured in the form of a small sized chip. Accordingly, the ion selective electrode is very advantageously employed for the determination of an electrolyte in a small amount of a liquid sample such as a body liquid. In most cases, a body liquid is available for the determination in a very limited amount. Moreover, the ion selective electrode is employable as a disposable device, because it can be formed in a simple structure and in a small size.
It has been noted, however, that a measured value obtained in the use of a small sized ion selective electrode sometimes is not reliable. This arises from fluctuation of electric potential (i.e., potential drift) which often takes place in the measurement procedure. It is thought that the potential drift is reduced by the use of an ion selective electrode in which the thickness of each of the functional layers is increased. However, the increase of the thickness of layers not only results in increase of cost for manufacturing the device, but also results in decrease of the sensitivity.
An improvement for obviating the occurrence of the potential drift or other disadvantageous problems has been proposed in Japanese Patent Provisional Publication No. 57(1982)-17852. This improvement comprises preparing a binderless electrolyte layer by vapor deposition of the electrolyte or by a series of procedures of coating an aqueous electrolyte solution containing no binder and then drying the coated layer. This art provides an ion selective electrode which is reduced in occurrence of the potential drift taking place in the conventional ion selective electrode. Nevertheless, more reduction of the possible potential drift is desired for the purpose of enhancing the accuracy of the meaurement in the use of an ion selective electrode.
It has been noted that the electrolyte layer prepared from an aqueous electrolyte solution containing no binder by a coating-drying procedure comprises relatively large sized crystalline electrolytes. Therefore, the crystalline elecrolytes are not distributed uniformly in the layer and the resulting electrolyte layer is apt to have a large thickness. The non-uniform electrolyte layer and thick electrolyte layer both serve to decrease the accuracy of measurement.
The preparation of an electrolyte layer by vapor deposition of an electrolyte is disadvantageous in an industrially available scale, because the vapor deposition of an electrolyte giving a low vapor pressure is difficultly employed, and otherwise the vapor deposition of an electrolyte decomposable at a vaporized temperature requires specific carefulness so that no high vaporization efficiency is expected.